Crossed-field mhd plasma generator/accelerator



Oct. 20., 1970 A. p, SABOL 3,535,586

cRossED-FIELD MHD PLASMA GENERATOR/ACCELERATOR Filed Jem.I 24. 1969 y 2sheets-sheet a Q @i 'l |3 l5` I7 s 2| FIG. 8 @E @5&7 @A9921 INVENTOR.

ALEXANDER P. sABol.

ATTORNEY United States Patent O M U.S. Cl. 315--111 10 Claims ABSTRACT FTHE DISCLOSURE This disclosure describes a crossed-field MHD plasmagenerator and/ or accelerator wherein a plurality of electrode pairs aremounted in a closed channel and a constant magnetic eld is appliedacross the channel. The sides of the rectangular shaped channel parallelto the electrode pairs forms an idler (electrically oating) electrode. Aworking gas is introduced at one end of the channel. In operation, amain arc, supplied by a continuous source of power, is created acrossthe pair of electrodes adjacent to the -point of gas introduction. As itis being acted on by MHD forces, the main arc moves to the next pair ofelectrodes and so on downstream. Each time a main arc moves to the nextpair of electrodes, side arcs are formed between each of the twoelectrodes of the prior electrode pair and the idler electrode. The sidearcs move upstream in the outer sections of the channel and reconnectthe electrode pair to form more main arcs. The reconnected main arcsthen move downstream because of being acted on by MHD forces. Due to anelectrical shorting action, the main arcs extinguish at the lastelectrode pair. The action of the main arcs accelerates plasma which isemitted from the channel.

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION The prior art discloses various types ofplasma accelerators and generators such as the crossed-field plasmagenerator, for example. One prior art crossed-field plasma generatorincludes a plurality of segmented electrodes mounted perpendicular to amagnetic field so as to provide a smooth channel. The sides of thechannel are electrically insulated from the surrounding environment. Theprimary disadvantage of a crossed-field plasma generator of this natureis that the arc discharge is created and remains attached to each pairof segmented electrodes during the entire operation of the generator.This arc or electrical discharge attachment causes two undesirableresults. First, severe erosion of the downstream face of the anodeelectrode and its adjoining insulator occurs. Secondly, blow-outfrequently occurs; that is, the shape of the discharge balloonsdownstream and bears against both electrode surfaces, resulting in poorefficiency.

Therefore, it is an object of this invention to provide a a new andimproved crossed-field plasma generator.

It is also an object of this invention to provide a new and improvedcrossed-held plasma generator that has improved erosion characteristicsover prior art crossedield plasma generators.

It is still a further object of this invention to provide a new andimproved plasma generator wherein blowout is less likely to occur thanin prior art plasma generators.

3,535,586 Patented Oct. 20, 1970 It is a still further object of thisinvention to provide a new and improved plasma generator thatcontinuously generates plasma out one end as gas in continuouslyinjected into the other end.

It is a still further object of this invention to provide a new andimproved plasma generator that has greater etliciency of operation thanprior art plasma generators.

SUMMARY OF THE INVENTION In accordance with a principle of thisinvention, a new and improved magnetohydrodynamic (MHD) cros sed-fieldplasma generator is provided. The generator comprises a plurality ofpairs of electrodes mounted in a channel. The channel forms an idlerelectrode. AS gas is injected into one end of the channel and, a mainarc, formed across a trst pair of electrodes, creates a plasma regionwhich ows down the channel as the main arc proceeds from electrode pairto electrode pair. The main arc moves due to the presence of an appliedmagnetic field. More specically, as the main arc moves downstream fromelectrode pair to electrode pair, it accelerates the plasma region withit. The main arc is extinguished at the last electrode pair and theaccelerated plasma passes out of the opposite end of the channel. As themain arc moves from electrode pair to electrode pair, secondary arcsform between each electrode of a prior electrode pair and idlerelectrode and move upstream to form a new main arc across the priorelectrode pair. There is a continuous movement of a plurality of mainarcs down the channel with each main arc accelerating a plasma regionwith it.

In accordance with a further principle of this invention, cooling meansare provided for cooling the electrodes and the portion of the channelforming the idler electrode and any other portion of the devicerequiring cooling.

In accordance with a still further principle of this invention, meansare provided for segmenting the idler electrode into a plurality ofsegments to prevent a Hall potential from axially shorting theelectrodes.

In accordance with a still further principle of this invention, meansare provided for controlling the application of electric potentials tothe idler electrode and the electrodes so that predetermined potentialsand electrode current flows are applied to those elements.

In accordance with an alternative principle of this invention, the idlerelectrode is insulated from the pairs of electrodes downstream from thelirst pair of electrodes so that secondary arcs only occur around thefirst pair of electrodes.

It will be appreciated from the foregoing summary of the invention thata new and improved MHD crossed-field plasma generator is provided. Thegenerator of the invention is more efficient than prior generators,because there is a continuous movement of plasma down the channel whichexhausts from the end of the channel. Hence, there is no interruption ofplasma generation as occurs in prior art devices. In addition, thedischarge does not remain xed to any particular spot on the electrodes;hence, electrode erosion is reduced. Further, because a multiplicity ofdischarges exist simultaneously, plasma acceleration. is improved.Moreover, undesirable blow-out or ballooning at any particular electrodeis reduced due to the proper extinguishment of the arc at the end of thechannel. Hence, the invention provides numerous advantages over priorart plasma generators. In addition, the invention is uncomplicated tomanufacture.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing objects and many of theattendant advantages of this invention will become more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 is a cross-sectional diagram of one embodiment of the invention;

FIG. 2 is a front view of the channel of the embodiment of the inventionillustrated in FIG. 1;

FIG. 3 is a cross-sectional diagram similar to the diagram of FIG. 1utilized to describe the operation of the invention;

FIG. 4 is a cross-sectional diagram of an alternative embodiment of theinvention.

FIG. 5 is a cross-sectional diagram of a portion of the embodiment ofthe invention illustrated in FIG. 4 illustrating one modificationthereof;

FIG. 6 is a cross-sectional diagram of a portion of the embodiment ofthe invention illustrated in FIG. 4 illustrating a second modificationthereof;

FIG. 7 is a cross-sectional diagram of a further embodiment of theinvention; and

FIG. 8 is a cross-sectional diagram of a still further embodiment of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a cross-sectionalview along lines 1 1 of FIG. 2 of one embodiment of the invention, andFIG. 2 is a front view along lines 2-2 of FIG. 1 of that embodiment. Theembodiment of the invention illustrated in FIGS. 1 and 2 comprises: aclosed channel 11; first through seventh electrode pairs 13-13, 15-15,17-17, 19-19, 21-21, 23-23, and 25-25; a working gas supply pipe 27; anelectrical direct current power supply 29, and a control 31. Theelectrode pairs 13-13 through 25--25 are long tubular members and aremounted in parallel at right angles to the axis of the channel 11 atspaced apart points. The spacing between the electrodes is preferablyequal, but precise spacing will depend upon operational requirements. Inaddition, the electrodes are equally spaced from the longitudinal axis12 of the channel 11, but can be altered to suit conditions. Moreover,the electrodes are approximately equally spaced from the sides of thechannel.

As viewed in FIG. 1, the channel has upper and lower sides 14 and 16that come together at a closed end 18 to form a generally U-shapedcross-section. This portion of the channel is made of a suitableelectrode material and forms, as hereinafter described, an idlerelectrode. The other end of the channel 11 is open. The closed end 18 ofthe channel is attached to the gas supply pipe 27 which supplies asuitable gas such as air, for example. A plurality of apertures 33 allowgas from the gas supply pipe 27 to flow into the closed end 18 of thechannel. The upper electrodes of the electrode pairs are connected by aconductor 35 to one side of the power supply 29. The other side of thepower supply 29 is connected to the control 31. The control 31 has aplurality of outputs that are connected to the lower electrodes of theelectrode pairs and to the idler electrode. It will be appreciated thatthis manner of connection is merely illustrative. That is, the powersupply could be formed of a plurality of separate independent powersupplies connected across the various electrode pairs. Alternatively,the power supply could be a single power supply connected via a resistornetwork to the various electrode pairs. The primary requirement is thatappropriate arc-creating voltages be applied across the variouselectrode pairs and between the electrode pairs and the idler electrode.

As best illustrated in FIG. 2, the two remaining sides of the channelare closed by insulating plates 37. Mounted outside of the insulatingplates are side plates 39. The

side plates have apertures through which the electrode pairs projectwith the electrodes being appropriately insulated from the side plates.Hence, the side plates and the insulating plates maintain the electrodesin their predetermined positions. Mounted outside of the side andinsulating plates are magnets 41 mounted so as to create acrossed-magnetic field. That is, a magnetic flux passes between themagnets so as to provide a magnetic field perpendicular to the channel11 as the channel is illustrated in FIG. 1. It will be appreciated thatthe magnets 41 are merely illustrative and can take on various forms.For example, they can be permanent magnets. It will further beappreciated by those skilled in the art that the direction of themagnetic field depends upon the polarity of the power applied to theelectrode pairs. If desired, the strength of the magnetic field can bevaried along the length of the channel by the use of any suitablecontrol means or construction.

Also illustrated in FIG. 1 is a fine wire 43 connecting the firstelectrode pair 13 13. This wire is placed across the first electrodepair 13-13 so that an initial arc can be created across that electrodepair when power is first applied to the invention. The wire 43 is suchthat it vaporizes when power is applied leaving an arc across the firstelectrode pair. It is to be noted that other suitable starting de- Vicescan also be employed.

Turning now to the operation of the invention: the control 31 appliespower to the electrode pairs, while a properly controlled gas, such asair, is introduced into the channel 11 from the gas supply pipe 27 viathe apertures 33 in the direction of the arrows. The application ofpower causes a current iiow through wire 43, which vaporizes, and anelectrically conducting plasma (arc) is formed across the firstelectrode pair 13-13. This arc or electrically conducting plasma movesin the direction of the second electrode pair 15-15 because of theexternally imposed magnetic field. The movement of the arc acceleratesgas and plasma with it in the direction of the arrows, i.e., from leftto right. More specifically, the arc shape balloons outwardly toward thesecond electrode pair 15-15 until it makes contact with that electrodepair, whereupon the power supply 29 discharges through that arc nowacross electrode pair 15-15. Thereafter, the main arc 45 is in theposition illustrated in FIG. 3, i.e., across the second electrode pair.

Because of the MHD forces, the -segments of the arc between both sidesof the first and second electrode pairs 13-13 and 15-15 move into thearea between the first electrode pairs 13-13 and the idler electrodesides 14 and 16. These side arcs illustrated at 43 in FIG. 3 move in thedirection of their arrows, i.e., toward the closed end 18 of the idlerelectrode. The arcs 43 continue to move until they join each other andmove into the position of the starting conductor 43. At this point, onecycle of arc rotation is completed for the first electrode pair 13-13.

Meanwhile, the first main arc 45 is ballooning from the second electrodepair 15-15 toward the third electrode pair 17-17. And, the same sequenceof events takes place between the second and the third electrode pairs.That is, the third electrode pair 17 as-sumes the burden of moving themain arc 45 down the channel and two separate side arcs are formed aboutthe second electrode pair 15. However, this time the side arcs formedabout the second electrode pair 15-15 join the second center arc thathas been born in the meantime across the first electrode pair 13-13.Thi-s process continues through the seven electrode pairs until the rstmain arc 45 reaches the end of the channel. The movement of the main arcaccelerates the plasma within the arc in the direction of the arrows. Atthe last electrode pair 425-25, the moving main arc balloons out to thechannel until the side arcs make contact with the idler electrode side14 and 16. When the Iside arcs are created, the main arc is extinguishedand only the plasma leaves the channel 11.

It will be appreciated by those skilled in the art and others that thenumber of electrodes in the channel is determined by the amount ofacceleration desired. Preferably, the channel is designed so as to takeadvantage or aerodynamic and MHD requirements of a particular use of theinvention. Hence, the invention is not limited to a generallyrectangular cross-sectional channel of the type illustrated in FIGS.l-3-the channel could have a conical cross-sectional shape, for example.

The sequence of events described above suggests that there are half asmany main arcs simultaneously moving down the channel at any one time asthere are numbers of electrode pairs. However, under some operatingconditions there may be the same number of main arcs as there areelectrode pairs. And, each discharge, because of MHD forces, acceleratespart of the flow down the entire working section of the channel. Inaddition, it should be noted that the side arcs create a back flow thataids in the acceleration of the plasma flow in the direction of thearrows, i.e., from left to right. This back ow enters the channel aboutthe electrode pair 13-13. It arrives there by passing through theregions between the electrodes on one side or the other as a consequenceof the side arc motion. This pumping action can be controlled by aproper adjustment of the operating parameters, i.e., the location of theelectrodes, the size of the channel, the viscosity of the incoming gas,etc.

It should also be noted that while the foregoing description hasdescribed both a gas and a plasma region about each arc as the main arcsmove down the channel, the channel can contain only plasma if there isno complete or sudden division between the main arc discharge and thesurrounding environment. Thus, the accelerator can operate from a plasmagenerator that replaces the first electrode pair 13-13.

It will be appreciated by those skilled in the art and others that theembodiment of the invention illustrated in FIGS. 1-3 will operatesatisfactorily in many environments. However, in some environments itmay have certain unsatisfactory characteristics. For example, theembodiment of the invention illustrated in FIGS. 1-3 may over-heat ifoperated continuously for long periods of time. Moreover, a Hallpotential may be created because the idler electrode is continuous alongthe length of the channel. The embodiments of the invention hereinafterdescribed overcome these disadvantages.

The embodiment of the invention illustrated in FIG. 4 comprises the gasinput pipe 27 and five pairs of electrodes, 13-13, 15-15, 17-17, 19-19and 21-21 mounted in a channel 11 generally similar to the channelillustrated in FIGS. 1-3. The primary difference between the embodimentillustrated in FIG. 4 and the embodiment illustrated in FIGS. 1-3 isthat the electrodes each include central apertures 51 through which acooling liquid, such as Iwater, for example, can flow. In addition, theidler electrode is broken into a plurality of sections and each sectionincludes an aperture 53 through which a cooling liquid can flow. Thesections are separated by insulating separating members 55 which may beformed of boron nitride, for example. However. while the idler electrodeis separated into a plurality of section, it will be appreciated thateach pair of opposite sections are electrically connected together sothat across the channel the idler electrode potential remains the same.The operation of the embodiment of the invention illustrated in FIG. 4is identical to the operation of the embodiment of the inventionheretofore described; hence. it will not be restated here.

FIGS. 5 and 6 illustrate modifications of the inner surface idlerelectrode sections suitable for use in the embodiment of the inventionillustrated in FIG. 4. Specifically, FIG. 4 illustrates that the inneror channel surface of the idler electrode -sections is fiat. FIG. 5illustrates that they can have a concave curvature, while FIG. 6illustrates that they can have a convex curvature. The exact flatness orcurvature utilized will depend upon the intended use of a specificembodiment of the invention.

FIG. 7 is a further alternative embodiment of the invention that issimilar to the FIG. 4 embodiment of the invention, with the differencethat the axes between the electrode pairs are not perpendicular to themain axis of the channel, as in the previously illustrated embodiments.Rather, the axes between the electrode pairs are angled with respect tothe main axis 12. Hence, the invention does not require a right angularrelationship with respect to the axis between the electrode pairs andthe main axis. The operation of the FIG. 7 embodiment of the inventionis identical to the previously described operation of FIGS. 1-3embodiment of the invention and will not be restated here.

FIG. 8 illustrates a still further alternative embodiment of theinvention that operates in a slightly different manner than thepreviously described embodiments. Specifically, the FIG. 8 embodimentcomprises five pairs of parallel electrodes 13-13, 15-15, 17-17, 19-19,and 21-21, mounted in a channel 11. The channel is generally U-shaped incross section. A plurality of apertures 33 are located in the end of theend section 57 of the U to allow gas to pass from the gas input pipe 27into the channel 11. Each leg of the U is broken by an insulating member59. Elements 61, located adjacent to the insulating members 59, projectoutwardly to define a portion of the channel 11. Located on the insideof the elements 61 to form the inner face of the channel are inuslatingsurfaces 63 which may be formed of a refractory material such as boronnitrite, as may the insulating members 59. The mounting is such that aflat surface for the channel 11 is provided. The section 57 and theelements 61 each include cooling channels 65 and 67, respectively,through which a cooling liquid may flow, if desired.

The operation of the embodiment of the invention illustrated in FIG. 8is generally similar to the operation of the previously describedembodiments, except that no secondary arc is formed around any of theelectrodes except the first electrode pair 13-13. The reason nosecondary arc is formed around the second, third, fourth, and fifthelectrode pairs is that the electrodes are mounted so that there is aportion of the insulating surface 63 located between each of them andthe elements 61 which, if not for the insulating surfaces, would form aportion of the idler electrode.

The embodiment of the invention illustrated in FIG. 8 is operative inenvironments where secondary arcs are undesirable or dangerous. Ingeneral, a first arc is created across the first electrode pair 13-13,which balloons outwardly to the second electrode pair 15-15. Secondaryarcs are created around the first electrode pair 13--13 and moveupstream to reform a new center or main arc across the first electrodepair 11i- 13, Meanwhile, due to the influence of the magnetic field, thefirst main arc located across the second electrode pair 15-15 balloonsoutwardly and forms an arc across the third electrode pair 17-17. Thisoperation continues until the first arc exhausts out the end of thechannel 11 after being electrically shorted out by the following secondarc. Other main arcs proceed down the electrode pairs in the samemanner.

It will be appreciated from the foregoing description that the inventionprovides a` crossed-field MHD plasma generator that accelerates plasmadown the channel, as the plasma is formed, due to the influence of amagnetic field. The invention has certain advantages over the prior artdevices. For example, the invention does not require a plasma generatoras an addition, as do prior art crossedfeld accelerators. Further, thedischarge or arc continuously movesdown the channel, thereby preventingany erosion of electrodes due to a constant arc existing across one pairof electrodes. In addition, a multiplicity of discharges existsimultaneously to accelerate the plasma. Moreover, the arcs do notballoon out at any one electrode or at the end electrode; rather, theyare properly extinguished at the last electrode pair.

What is claimed is:

1. A crossed-held plasma generator/ accelerator comprising:

a channel defined by an idler electrode on two opposing sides and byinsulating plates on the other two opposing sides;

a plurality of pairs of cylindrical electrodes mounted in apredetermined spaced relationship in said channel;

a magnetic means for applying a crossed magnetic field to said channel;and

power supply means connected to said plurality of pairs of electrodesand to said idler electrode for energizing said plurality of pairs ofelectrodes and said idler electrode.

2 A crossed-eld plasma generator/ accelerator as claimed in claim 1including gas supply means for applying a gas to one end of said channel3. A crossed-field plasma generator/accelerator as claimed in claim 2,wherein said end comprises an end member having apertures therein andwherein said gas supply means includes a pipe connected to said endmember whereby gas, introduced by said gas supply means, is allowed topass into said channel.

4. A crossed-field plasma generator/accelerator as claimed in claim 3,wherein the two opposing sides of said channel forming said idlerelectrode are sectioned and including insulating means mounted betweensaid sections for insulating said sections from one another.

5. A crossed-field plasma generator/ accelerator as claimed in claim 4including cooling means forming a 8 part of said idler electrode forcooling said idler electrode.

6. A crossed-field plasma generator/accelerator as claimed in claim 5,wherein said cooling means includes apertures in said idler electrode toallow a cooling fluid to ow through said idler electrode.

7. A crossed-field plasma generator/accelerator as claimed in claim 6including apertures in said plurality of pairs of electrodes to allow acooling `fluid to flow through said electrodes.

8. A crossed-field plasma generator/accelerator as claimed in claim 7,wherein the inner surface of said idler electrode sections are concave.

9. A crossed-field plasma generator/accelerator as claimed in claim 7,wherein the inner surface of said idler sections are convex.

10. A crossed-field plasma generator/accelerator as claimed in claim 7including insulating means for insulating the inner sides ofpredetermined sections of said idler electrode for a predetermineddistance along the length of said channel.

References Cited UNITED STATES PATENTS 2,614,232 10/1952 Kalb 313-1563,171,060 2/1965 Wood et a1. 313-231 X 3,333,152 7/1967 Sabol 315-111JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner U.S.Cl. X.R.

